Coin handling devices of the foregoing types have employed eddy current sensors to discriminate among various coins. Note that the term "coin" is broadly used in this specification and includes any type of coin, token or object substituted therefor. An eddy current sensor includes at least one primary coil for inducing eddy currents in the coin to be analyzed. The primary coil receives an alternating voltage which correspondingly produces an alternating current in the coil. The alternating current flowing in the primary coil produces an alternating magnetic field through and around the coil as is well known in the art.
Characteristics of the alternating magnetic field depend upon a variety of factors including the frequency and amplitude of the voltage applied to the primary coil, more fully described below. The primary coil, also know as the excitation coil, inductively couples with a coin brought into proximity with the coil, thereby producing eddy currents in the coin being analyzed. Because the magnetic field from the primary coil is alternating, the corresponding eddy currents are alternating as well. The induced eddy currents are influenced by the material composition of the coin being analyzed.
The alternating eddy currents induced in the coin also produce magnetic fields of their own. These magnetic fields are detected with one or more secondary coils, also known as detection coils. Because eddy current sensors take on a transformer-like configuration, with primary and secondary coils, the primary coil also induces an alternating voltage on the secondary coil or coils. The voltage induced on the secondary coil or coils from the primary coil can be described as a common-mode voltage and must be eliminated or ignored in order to focus on the eddy current signal made up of much smaller voltages induced on the secondary coil by the eddy currents. This has previously been accomplished by processing the voltage signal from the secondary coil to eliminate the voltage induced on the secondary coil by the primary coil. Such signal processing can have the undesirable effect of increasing the number of components in the system, which correspondingly increases signal distortion and the possibility of other problems such as part failure, electrical noise and manufacturing complexity. Such signal processing may also decrease the ability to resolve fine variations in the eddy current signal.
The strength of the eddy currents produced is directly affected by the frequency of the alternating magnetic fields applied. A tradeoff exists between the use of high and low frequencies in coin discrimination. High frequencies tend to create magnetic fields that penetrate less deeply into the coin, thus making surface composition and structure more important. This can become disadvantageous when discriminating among cladded coins with designs on one or both sides. Low frequencies tend to penetrate further into the coin, giving a better indication of overall composition, but have the disadvantage of increased likelihood of causing spurious signals in material surrounding the coin in the coin handler because of the more extensive penetration of the magnetic field.
Prior art eddy current sensors have tended to be large in order to produce large magnetic fields. Coin handlers employing multiple eddy current sensors can experience cross-talk between sensors. Unfortunately, cross-talk interferes with accurate determination of coin material content.